This application is based on Japanese Patent Application No. 11-237299 (1999) filed Aug. 24, 1999, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a printing method and a printing apparatus, and is particularly suited for adjusting the positions of ink dots in a printing apparatus of an ink jet system. In addition to general printing apparatus, the present invention can also be applied to copying machines, facsimiles with a communication system, word processors with a printer, and industrial printing apparatus combined with a variety of processing devices.
2. Description of the Related Art
An image printing apparatus of so-called serial scan type, which executes the print operation while scanning a print head, or a printing unit, over a print medium, has found a variety of image forming applications. The ink jet printing apparatus in particular has in recent years achieved high resolution and color printing, making a significant image quality improvement, which has resulted in a rapid spread of its use. Such an apparatus employs a so-called multi-nozzle head that has an array of densely arranged nozzles for ejecting ink droplets. Images with still higher resolution has now been made possible by increasing the nozzle density and reducing the amount of ink per dot. Further, to realize an image quality approaching that of silver salt picture, various technologies have been developed, including the use of pale or light color ink with reduced density in addition to four basic color inks (cyan, magenta, yellow and black). A print speed reduction problem, which is feared to arise as the picture quality advances, is dealt with by increasing the number of print elements, improving the drive frequency and employing a bi-directional printing technique, thus realizing a satisfactory throughput.
FIG. 19 schematically shows a general construction of a printer that uses the multi-nozzle for printing. In the figure, reference number 1901 represents head cartridges corresponding to four inks, black (K), cyan (C), magenta (M) and yellow (Y). Each head cartridge 1901 consists of an ink tank 1902T filled with a corresponding color ink and a head unit 1902H having an array of many nozzles for ejecting the ink supplied from the ink tank onto a print medium 1907.
Designated 1903 is a paper feed roller which, in cooperation with an auxiliary roller 1904, clamps a print medium (print paper) 1907 and rotates in the direction of arrow in the figure to feed the print paper 1907 in the direction as required. Denoted 1905 is a pair of paper supply rollers that clamp the print paper 1907 and carries it toward the print position. The paper supply rollers 1905 also keep the print paper 1907 flat and tight between the supply rollers and the feed rollers 1903, 1904.
Designated 1906 is a carriage that supports the four head cartridges 1901 and moves them in a main scan direction during the print operation. When the printing is not performed or during an ink ejection performance recovery operation for the head unit 1902H, the carriage 1906 is set at a home position h indicated by a dotted line.
The carriage 1906, which was set at the home position h before the print operation, starts moving in the X direction upon reception of a print start command and at the same time the head unit 1902H ejects ink from a plurality of nozzles (n nozzles) formed therein according to print data to perform printing over a band of a width corresponding to the length of the nozzle array. When the printing is done up to the X-direction end of the print paper 1907, the carriage 1906 returns to the home position h in the case of one-way printing and resumes printing in the X direction. In the case of bi-directional printing, the carriage 1906 also performs printing while it is moving in a xe2x88x92X direction toward the home position h. In either case, after one print operation (one scan) in one direction has been finished before the next print operation is started, the paper feed roller 1903 is rotated a predetermined amount in the direction of arrow in the figure to feed the print paper 1907 in the Y direction a predetermined distance (corresponding to the length of the nozzle array). By repeating the one-scan print operation and the print paper feeding by a predetermined distance, data for one sheet of paper is printed.
In the above serial type ink jet printer, various provisions have been made as to the construction of the head unit or the printing method in order to realize an image printing with higher resolution.
For example, the manufacture of the multi-nozzle head inevitably places a limit on the density of the nozzles in a single nozzle array.
FIG. 20A shows an example head that realizes a higher nozzle density. This head has two columns of nozzles extending in the Y direction and spaced a distance px (corresponding to a predetermined number of pixels) apart in the X direction. The two nozzle columns, each consisting of many nozzles arranged at a predetermined pitch py in the Y direction, are shifted from each other by a distance py/2 in the Y direction. This arrangement of the nozzles realizes a resolution two times higher than that achieved by a single nozzle column. When this head is applied to the apparatus shown in FIG. 19, the heads having the construction shown in FIG. 20A for one color can be arranged in parallel in the X direction for six colors. In this arrangement, simply adjusting the ejection timings of the two nozzle columns can achieve a color printing with two times the resolution of the single nozzle column.
In other technologies, such as U.S. Pat. No. 4,920,355 and Japanese Patent Application Laid-open No. 7-242025 (1995), a high resolution printing is realized by setting the paper feed distance for each print scan to a predetermined number of pixels less than the length of the column of nozzles while leaving the multi-nozzle arrangement at a low resolution. Such a printing method is hereinafter called an interlace printing method.
The interlace printing method will be briefly explained by referring to FIG. 21. Here let us take up an example case where an image with resolution of 1200 DPI (dots/inch) is printed by using a head H with nozzles arranged at a pitch of 300 DPI. For the sake of simplicity, it is assumed that the head has nine nozzles and that the distance of the paper feed carried out after each print scan is nine pixels at 1200-DPI resolution. The rasters printed in the forward pass are shown as solid lines and the rasters printed in the backward pass are shown as dashed lines. These two kinds of lines are formed alternately.
While in this example the paper is fed a fixed distance of 9 pixels at 1200-DPI resolution, other arrangements may be made in the interlace printing. The interlace printing method does not need to have a constant paper feed distance at all times as long as a picture is printed with a plurality print scans arranged at a pitch finer than the arrangement pitch of the nozzles themselves. In either base, an image can be printed with a higher resolution than he nozzle arrangement resolution.
By the various methods explained above, it has been realized to print an image with resolutions higher than a nozzle array.
On the other hand, a printing resolution of a printer is not necessarily equal to an input resolution from a host device as an image data supply source. Recent printers permit printing according to plural input resolutions. For example, when high speed processing is desired, it is possible even for a printer with a resolution of 1200 DPI to reduce a transmission time from the host device if an image data is inputted with a resolution of 300 DPI. In such a case, 17 levels which can be expressed by 4xc3x974 pixels, in practice, are reduced to 2 levels.
Moreover, even if a printing of high quality image is required, it becomes a burden for the host device to transmit an image data of 1200 DPI. In this case, a method has already been proposed and brought into practice, wherein a multi-valued image is inputted beforehand with a resolution of 600 DPI, data amount thereof is a quarter of that of 1200 DPI, and is represented in multi-levels in 2xc3x972 areas for printing. As one of the examples thereof, the technique disclosed in the Japanese Patent Application Laid-open No. 9-466522 (1997) will be explained below.
FIG. 22 shows an example disclosed in the specification of the above mentioned Application wherein an input data with a resolution of 300 DPI is printed with 800 DPI. In the case that an output resolution of a printer is 600 DPI to an input resolution of 300 DPI from the host device, the printer can represent the output in 5-valued gradations by 2xc3x972 dot arrangement. When the 5 values are defined as xe2x80x9clevelxe2x80x9d to xe2x80x9clevel 4 xe2x80x9d, plural patterns can be represented in each of the levels except xe2x80x9clevel 0xe2x80x9d and xe2x80x9clevel 4xe2x80x9d as shown in the figure. The Japanese Patent Application Laid-open No. 9-46522 (1997) discloses the contents how these plural patterns are arranged sequentially or at random. In such a manner, a dot arrangement for forming a pixel at each gradation is not fixed, therefore, this method has an effect to reduce an pseudo-contour or what is called xe2x80x9csweep-up phenomenonxe2x80x9d etc. appearing on the edge part of an image when pseudo half-tone processing is carried out. Moreover, this printing method is also effective to average the use conditions of the printing head nozzles.
Such a printing method is effective especially for a high resolution printing device. For a printer intended for realizing a photographic image, an input resolution higher than a degree of visual resolution is not necessary, but it is rather effective to improve tone of individual pixels as far as a resolution of about 600 DPI can be obtained. If the tone is further increased by using 6 color inks containing the aforementioned light inks, granular impression is decreased and a smoother image can be obtained.
As opposed to this, when an output of a fine monochrome character or pattern is desired, it is preferable to print a 2-valued image at a degree of an input resolution as the highest resolution of the printer. Thus, it becomes possible to cope with various prints according to uses while using the same printer.
When a head as shown in FIG. 20A is used, because even-numbered rasters and odd-numbered rasters that are alternated in the Y direction (sub-scan direction) are printed by different columns of nozzles, the landing positions of ink droplets from the two columns of nozzles may deviate subtly from the correct positions, degrading the image quality. One of the possible causes for this problem may be explained as follows. When a head face on which nozzles are formed is deformed due to swelling with ink or temperature rise, causing a part of the head face between the nozzle column associated with the odd-numbered rasters and the nozzle column associated with the even-numbered rasters to bulge, as shown in FIG. 20B, the ink droplets from the respective nozzle columns will be projected in two different directions slightly away from each other. The ink landing position deviation between the rasters due to this phenomenon, even if small in magnitude, will have bad effects on the image quality, and a fine image obtained by using the binarization method like an error diffusion method is particularly deteriorated in the quality. Such an image is keeping the whole smoothness by being provided with a blue noise characteristic in the dot arrangement, however, the deviation between a layer of the even-numbered rasters and a layer of the odd-numbered rasters from each other largely collapses the spatial frequency distribution, and this produces a granular impression on the whole image.
Many proposals have been put forward as to the method of correcting ink landing position deviations among different colors and, in the bi-directional printing, the method of correcting deviations in ink landing position of the same color between the forward scan and the backward scan. However, as for the correction of the ink landing position deviations between the rasters of the same color produced by the head shown in FIG. 20A, an effective adjustment method has yet to be proposed although the allowable range for the deviation is narrow and the effects of such the deviations on the image formation are large.
Further, the deviation in ejection direction between the even-numbered nozzle column and the odd-numbered nozzle column is caused by the ink composition, ink ejection history such as ejection frequency, and printing environment, as well as the characteristic variations of individual heads. For example, when printing is done successively, such a phenomenon is brought about as the printing head is increased in temperatures and the inks re decreased in the viscosity, and this results in an increase in an ink ejection speed. As the head is raised in temperatures, so the deviation amount also varies according to the temperature rises, however, since the variation returns to the former state when the printing is completed and the temperature is lowered, it has been impossible to follow this variation even though an adjusting mechanism by user is provided.
Such a problem may occur in the above interlace printing method. In the interlace printing method, because the same image area is completed by repeating the print scan and the paper feed a plurality of times, the printing time will increase. To cope with this problem, a bi-directional printing has been proposed and disclosed. In this case, the odd-numbered rasters are often printed by the forward scans and the even-numbered rasters by backward scans, as shown in FIG. 21. If the ink landing positions deviate from one raster to another, the similar problem to that when the head of FIG. 20A is used will occur.
Although many methods for correcting the landing position deviation between forward printing and backward printing were already proposed, they have been still incomplete in timely and properly coping with the problem that the ejection speed increases as the temperature rises.
If the technique disclosed by the aforesaid Japanese Patent Application Laid-open No. 9-46522 (1997) is applied, the problem of deterioration in image quality can be relaxed to some extent. However, the proposal has not originally been intended for improving the landing position deviation between even- and odd-numbered rasters, and satisfactory position adjustment for forming dots between even- and odd-numbered rasters has not been examined.
Moreover, the above mentioned Application has also disclosed the contents for varying plural patterns at random. If this proposal is applied to the above problem, an effect can surely be expectable, however, circuitry for generating plural patterns at random becomes necessary, and moreover, it is anticipated that the circuitry will be rather complex. Although the above mentioned Application proposes to generate plural patters at random, as long as there is a limit to a memory for supplying the plural patterns, it is also presupposed that a large cycle appears in the patterns in view of an entire image, and is rather conspicuous.
The present invention is made considering the above-mentioned problems, and the purpose of the invention is to make it possible to suppress deterioration in image quality caused when dot forming positions are deviated from each raster, while using a head comprising plural ejection openings arranged in the main scanning direction and permitting high resolution printing, or while bi-directionally executing an interlace printing method.
In a first aspect of the present invention, there is provided a printing method using a print head on which a plurality of print elements are arranged, the method comprising the steps of:
forming an image on a printing medium by scanning the print head in a direction different from the arranging direction of the plurality of print elements and forming adjoining N rasters of the image in the arranging direction under different conditions; and
allocating input image data quantized into multi-valued levels to Nxc3x97N dot arrangements as pseudo half-tone processing before the image formation; the allocating step having the steps of:
applying a plurality of the dot arrangements mutually different from each other to a same level of the input image data;
cyclically changing the dot arrangement in the scan direction according to predetermined rules, and
equalizing the number of dots of the adjoining N rasters in the one cycle.
In a second aspect of the present invention, there is provided a printing apparatus using a print head on which a plurality of print elements are arranged, the apparatus comprising:
a unit for forming an image on a printing medium by scanning the print head in a direction different from the arranging direction of the plurality of print elements and forming adjoining N rasters of the image in the arranging direction under different conditions; and
a unit for allocating input image data quantized into multi-valued levels to Nxc3x97N dot arrangements as pseudo half-tone processing before the image formation; the allocating unit applying a plurality of the dot arrangements mutually different from each other to a same level of the input image data, cyclically changing the dot arrangement in the scan direction according to predetermined rules, and equalizing the number of dots of the adjoining N rasters in the one cycle.
In a third aspect of the present invention, there is provided a control method of a printing apparatus for printing by using a print head on which a plurality of print elements are arranged, the method comprising the steps of:
controlling the print head to form adjoining N rasters of an image in the arranging direction of the plurality of print elements under different conditions when the image on a printing medium is formed by scanning the print head in a direction different from the arranging direction; and
allocating input image data quantized into multi-valued levels to Nxc3x97N dot arrangements as pseudo half-tone processing before the image formation; the allocating step having the steps of:
applying a plurality of the dot arrangements mutually different from each other to a same level of the input image data;
cyclically changing the dot arrangement in the scan direction according to predetermined rules, and
equalizing the number of dots of the adjoining N rasters in the one cycle.
In a fourth aspect of the present invention, there is provided a program for performing a control method of a printing by means of a computer, the apparatus for printing by using a print head on which a plurality of print elements are arranged, the method comprising the steps of:
controlling the print head to form adjoining N rasters of an image in the arranging direction of the plurality of print elements under different conditions when the image on a printing medium is formed by scanning the print head in a direction different from the arranging direction; and
allocating input image data quantized into multi-valued levels to Nxc3x97N dot arrangements as pseudo half-tone processing before the image formation; the allocating step having the steps of:
applying a plurality of the dot arrangements mutually different from each other to a same level of the input image data;
cyclically changing the dot arrangement in the scan direction according to predetermined rules, and
equalizing the number of dots of the adjoining N rasters in the one cycle.
In any one of the above aspects, in the equalization, differences in a covering ratio of the printing medium with formed dots may be put within 10% or less in a range where the adjoining N rasters are deviated from each other by at least xc2x12 pixels in the scan direction.
The print head may have N columns of print elements arranged side by side in the scan direction, the N columns of print elements may be shifted from each other by an amount less than a pitch at which the print elements are arranged in the column, and the N columns of print elements may print the N rasters.
The print head may be operated to scan the printing medium in the forward and backward directions, and image formation may be carried out by relatively transporting the printing medium in a direction perpendicular to the scan directions by quantities for printing at a density higher than the arranging density of the plurality of print elements between the forward and backward scanning, and the printing of the N(=2) rasters may be carried out by the scanning in the forward and backward directions.
The above method or apparatus may further comprises a step of or means for carrying out an adjustment of drive timings to the plurality of print elements in the N rasters.
The print head may be the one which carries out printing by ejecting ink, and the print elements may have ejection openings for ejecting the ink.
Here, the print head may have heating elements to generate thermal energy for causing film boiling in ink as an energy for ejecting ink from the ejection openings.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.